Weekly Notes from the Yohkoh Soft X-Ray Telescope

(Week 22, 2002)

Science Nugget: May 31, 2002

The Yohkoh hard X-ray flare catalog

Introduction

With the end of Yohkoh's scientific observations, the next step is to
organize the data in such a way that future researchers can make best use of
the entire database.
In fact, all too few systematic surveys have been done, deferred often by the
need to follow up quickly on the sort of spectacular single event that one
can see many examples of in these nuggets.
But theories guided by limited samples may err badly, so we really need to get
a proper overview.
The Yohkoh database, ten years plus and covering all phases of solar
activity, provides an ideal basis for such work.

The first of the four Yohkoh instruments to have come through with a
survey catalog is the Hard X-ray Telescope
HXT,
which observes in the energy range 13-93 keV.
This is nominally the domain of bremsstrahlung
radiation produced by non-thermal electrons in the quasi-relativistic energy
range.
Its low-energy channels also sometimes respond to thermal sources
closely related to those seen by Yohkoh's Soft X-ray Telescope
(SXT).
For several reasons HXT presented us with a revolutionary new kind of
observation.
Hard X-ray imaging now continues with the
RHESSI satellite.
A further point regarding HXT in the context of survey-type studies: the gain
calibration of the instrument was meticulously maintained over its lifetime,
because the imaging required it.

The catalog contents

The
catalog itself resides at Montana State University (on the Web), along with
definitions
and
sample images.
We have gone into the catalog for a
specific event
as an illustration, as discussed in the next section.
The whole catalog can be summarized in the histogram below (click on it to
see a larger and more beautiful version prepared by K. Yosimura for an ISAS
New Year's card):

The catalog contains 3,115 flares and the plot above shows the distribution
of major (red) and minor (blue) flares across the decade of observations,
spanning about one 11-year sunspot cycle centered on a minimum.

A sample event

Skimming through the catalog, a
nice event
met the eye.
Why? Because of its hard spectrum - a glance at the peak counting rates
from the four HXT energy channels immediately estimates the hard X-ray
spectrum.
This is because the selected channel widths result in equal counts for
a very hard spectrum (power-law index approximately 2, near the physical
limit).
The chosen event has this property and also does not have a "broken power
law", which many flares do.
Does such a flare follow the
Neupert effect
or is the spectral hardness only an extreme case of the normal development?
The plot below

shows that it's normal, if extreme in its spectral properties
(see also the
full time-series plot from the catalog).
The hard X-ray counts (dotted line) appear to differentiate the light
curve of soft X-rays, in rather good detail.
The image shows an extremely compact soft X-ray source and a bare hint of
double footpoints in hard X-rays.
The hard spectrum therefore corresponds to an event in the low corona,
an association familiar to the
observers but without a published analysis; also there is no theory
for it yet (of course!).

How to use the catalog

We went into the catalog with a specific question, curious about a single
event.
This is a good way to use it, but (as pointed out earlier)
a better use of the catalog would be for systematic surveys.
In
SolarSoft
there are tools for searching the catalog for specific attributes of flares;
there is also software for reproducing and extending the sample images in
the catalog.